While ἀρνίον is technically the diminutive of ἀρήν or ἀμνός, this distinction was largely lost. Since the Book of Revelation was almost certainly composed well after the rest of the NT, it is hardly surprising that its choice of ἀρνίον completely supplants the other forms.
This lexical shift/evolution of Greek is documented by both BDB and W E Vine which I reproduce in full below.
1: ἀρήν (Strong's #704 — Noun Masculine — aren — ar-ane' ) a noun the
nominative case of which is found only in early times, occurs in Luke
10:3 . In normal usage it was replaced by arnion (No. 2), of which it
is the equivalent.
2: ἀρνίον (Strong's #721 — Noun Neuter — arnion — ar-nee'-on ) is a
diminutive in form, but the dimunutive force is not to be pressed (see
Note under No. 3). The general tendency in the vernacular was to use
nouns in ---ion freely, apart from their dimunitive significance. It
is used only by the Apostle John, (a) in the plural, in the Lord's
command to Peter, John 21:15 , with symbolic reference to young
converts; (b) elsewhere, in the singular, in the Apocalypse, some 28
times, of Christ as the "Lamb" of God, the symbolism having reference
to His character and His vicarious Sacrifice, as the basis both of
redemption and of Divine vengeance. He is seen in the position of
sovereign glory and honor, e.g., John 7:17 , which He shares equally
with the Father, John 22:1,3 , the center of angelic beings and of the
redeemed and the object of their veneration, e.g. John 5:6,8,12,13;
15:3 , the Leader and Shepherd of His saints, e.g., John 7:17,14:4 ,
the Head of his spiritual bride, e.g., John 21:9 , the luminary of the
heavenly and eternal city, John 21:23 , the One to whom all judgement
is committed, e.g., John 6:1,16; 13:8 , the Conqueror of the foes of
God and His people, John 17:14; the song that celebrates the triumph
of those who "gain the victory over the Beast," is the song of Moses
... and the song of the Lamb, 15:3. His sacrifice, the efficacy of
which avails for those who accept the salvation thereby provided,
forms the ground of the execution of Divine wrath for the rejector,
and the defier of God, John 14:10; (c) in the description of the
second "Beast," Revelation 13:11 , seen in the vision "like a lamb,"
suggestive of his acting in the capacity of a false messiah, a
travesty of the true. For the use in the Sept. see Note under No. 3.
3: ἀμνός (Strong's #286 — Noun Masculine — amnos — am-nos' ) "a
lamb," is used figuratively of Christ, in John 1:29,36 , with the
article, pointing Him out as the expected One, the One to be well
known as the personal fulfilment and embodiment of all that had been
indicated in the OT, the One by whose sacrifice deliverance from
Divine judgment was to be obtained; in Acts 8:32 (from the Sept. of
Is. 53:7) and 1 Peter 1:19 , the absence of the article stresses the
nature and character of His sacrifice as set forth in the symbolism.
The reference in each case is to the lamb of God's providing, Genesis
22:8 , and the Paschal lamb of God's appointment for sacrifice in
Israel, e.g., Exodus 12:5,14,27 (cp. 1 Corinthians 5:7 ).
Note: The contrast between arnion and amnos does not lie in the diminutive character of the former as compared with the latter. As has
been pointed out under No. 2, arnion lost its diminutive force. The
contrast lies in the manner in which Christ is presented in the two
respects. The use of amnos points directly to the fact, the nature and
character of His sacrifice; arnion (only in the Apocalypse) presents
Him, on the ground, indeed, of His sacrifice, but in His acquired
majesty, dignity, honor, authority and power. In the Sept. arnion is
used in Psalm 114:4,6; in Jeremiah 11:19 , with the adjective akakos,
"innocent;" in Jeremiah 27:45 , "lambs." There is nothing in these
passages to suggest a contrast between a "lamb" in the general sense
of the term and the diminutive; the contrast is between "lambs" and
sheep. Elsewhere in the Sept. amnos is in general used some 100 times
in connection with "lambs" for sacrifice.